Equilibrium modelling of interferences in the visible spectrophotometric determination of aluminium(III): Comparison of the chromophores chrome azurol S, eriochrome cyanine R and pyrocatechol violet, and stability constants for eriochrome cyanine R-alumin

Equilibria in the Al3+H+-eriochrome cyanine R (ECR) system were studied by potentiometry and spectrophotometry in 0.10 M KC1 at 25 °C and p[H+]2.5–6.5. Potentiometric titration data were interpreted in terms of a monomeric 1:1 complex AlH−1L (log β1–1,1 = 1.75), an excess metal polymer Al3H2−2L2 (log β3–2,2 = 13.44), and excess ligand polymers with stoichiometries A14H−3L5 and A14H−4L5 (log β4−3,5 = 29.07; log β4− 4,5 = 25.30). For titrations involving more rapid addition of alkali there was evidence for an additional polymeric conjugate base, Al4H−5L5 (log β4 − 5,5 = 20.67). The ECR-aluminium stability constants, along with literature values for those of chrome azurol S (CAS) and pyrocatechol violet (PCV), were used to calculate interference effects in the spectrophotometric determination of Al. Calculations established that interference by fluoride, citrate, oxalate and salicylate in aluminium assays was a minimum for PCV at pH 6.5 and maximum attainable chromophore concentration. CAS and ECR were more subject to interference and had pH values of minimum interference which varied with the interferent according to its acid-base properties. The use of mass action effects to suppress interferences was limited by the comparatively high ligand-only absorbances for CAS and ECR.